I really don't give it any thought, just tend to start with the eccentric's max throw at 90+30deg ahead of the crank and see what it runs like from there, seldom needs further adjustment. This is for display on air not steam. Do have a couple with reversers so you can notch them up or down if you want.

But find it's easiest to set the compressor regulator to a low sub 5psi setting and use an inline valve to regulate air volume.

I I were to give Howards two example engines some thought then they really are at opposite ends of the "model" spectrum.

The Williamson is a detailed replica of an actual engine which even at full size was quite a slow runner at 130rpm. It therefore has a long stroke in relation to the bore 1 3/8" x 5/8" and is unlikely to be expected to do any significant work. I expect most builders of this would aim for 40rpm or less for display

The No4 on the other hand was never designed to represent a specific prototype and at the time it was first introduced was far more likely to have been expected to do real work. It's meant to run at far higher speeds and the 1.5" bore by 1 1/4" stroke are far more suited to this than the long stroke of the Williamson. A lot more of these models get built and used to drive something so would expect them to be run faster, even for display they may not run as slowly as the williamson,